PROJECT SUMMARY Liver cancer ranks fifth in frequency and third in mortality, with estimated numbers of over 700,000 new cases every year worldwide. Hepatocellular carcinoma (HCC) is the most common form of liver cancer that originates from viral infection (e.g., hepatitis B, C) or injury-driven chronic inflammation (e.g., cirrhosis from excessive alcohol consumption or obesity-induced steatohepatitis) in the liver. Although HBV and HCV incidence is on the decline, the obesity epidemic has resulted in an increase in the number of new cases of HCC in developed countries including the US. Thus, identifying targetable mediators of liver inflammation and HCC represents an important unmet medical need. Interleukin (IL)-27 is a recently discovered cytokine with immunomodulatory roles in infection and autoimmunity. IL-27R signaling reduces inflammation in murine models of atherosclerosis and experimental encephalomyelitis (EAE), ostensibly by suppressing a pro-inflammatory immune response. These findings led us to speculate that IL-27 might function similarly to limit liver inflammation and halt HCC development. Surprisingly, however, we have discovered that eliminating IL-27R signaling decreases tumor development in a mouse model of HCC, suggesting a new, pro-tumorigenic role in HCC for this otherwise anti-inflammatory cytokine. Here we propose to investigate the mechanism(s) by which IL-27R signaling promotes development of HCC and to test the efficacy of IL-27 blockade as a new therapeutic approach for this malignancy. To do so, we will use IL-27-deficient (Il27ra-/-) mice in an established carcinogen (diethylnitrosamine, or DEN)-induced model of liver cancer. We hypothesize that IL-27R signaling alters the balance between pro- and anti-tumor immune responses in HCC, and will define the cellular mechanisms by which IL-27 mediates these effects. We will elucidate how IL-27R signaling regulates immune cell activation and function during HCC development by integrating an array of genetic modeling, histological, cutting-edge imaging and molecular biological methods. Finally, we will test the efficacy of IL-27 blockade, either alone or in combination with other immunotherapies, as a new therapeutic avenue for HCC. Overall, the proposed research will uncover the role of IL-27R signaling in HCC development. This work has strong translational potential with game-changing ramifications for HCC.